Recent advances in digital signal processing technology have made it possible to efficiently encode large quantities of digital information such as moving and still pictures and video and to record the encoded information on a small-size magnetic medium or to transmit it to a communication medium.
A technique using the discrete wavelet transform is known as a highly efficient method of encoding an image. In accordance with this technique, the discrete wavelet transform is applied to an input image signal to be encoded. In the discrete wavelet transform, two-dimensional discrete wavelet transform processing is applied to an input image signal, and then a sequence of coefficients obtained by the discrete wavelet transform is quantized.
In such quantization, a region of an image to be encoded to an image quality higher than that of a peripheral portion of an image containing the image region is designated by a user. The coefficients that belong to the designated region are then evaluated, these coefficients are quantized upon raising the precision of quantization a prescribed amount, and encoding is carried out in such a manner that the designated image region can be decoded to an image quality higher than that of the periphery.
With this conventional technique, however, the designation of the image region desired to be encoded to a high image quality is an explicit designation made by the user. The operation demanded of the user is therefore a complicated one.
Further, if it is so arranged that the image region to thus be encoded to a high image quality is determined by automatically discriminating the patterns or colors of this image, a limitation is imposed on the colors or shapes of objects to be encoded to the high image quality and it will not be possible to obtain an object that can be used universally. For example, in a case where video shot by a home digital video camera or the like is to be processed, satisfactory results are not obtained.
Further, the specification of Japanese Patent Application Laid-Open No. 10-145606 describes a region discrimination method as a technique through which a wavelet transform is applied to an input image and a region of interest in the image is extracted using subband signals that are obtained. According to the invention described in this publication, separation of an image region is implemented depending upon whether a wavelet coefficient obtained by applying a Harr wavelet transform to an image signal, i.e., the absolute value of the high-frequency component of the subband signals, is greater than a predetermined threshold value.
With this example of the prior art, however, the purpose is to separate a region having a strong edge from a region having a weak edge by referring to the absolute values of the wavelet coefficients (i.e., of the subband signals). The segmentation of a higher-order multilevel region or the extraction of a region of interest, namely the extraction of a subject of interest from an image region, cannot be carried out.